This invention relates to a photoelectric smoke detector and more particularly to a spot type photoelectric smoke detector of the reflected light type.
Photoelectric smoke detectors are classified into two types, that is a direct incident light type or light transmission type which detects the variation in the quantity of the light received by the detector which is caused by the interception of the light from a light source by the smoke to be detected and a reflected light type in which the light emanated from a light source and reflected by the smoke to be detected is detected by a light receiving element. Of these two types, the former is not used as spot type smoke detector because the variation in the quantity of light between normal condition and emergency is small and the S/N ratio of the detector is poor. Although such disadvantages can be alleviated by making long the light path it is difficult to adopt this solution for the spot type detector which is designed to be installed in a limited space. Usually a pulsed lighting system is used for the light source because with the pulsed lighting system it is possible to pass a large current through the light source without increasing power consumption. In the light transmittion type it is necessary to synchronize the operation of the light source and the light receiving element. In the reflected light type such synchronization is also used but is not always necessary.
In the reflected light type it is necessary to preadjust the quantity of the light incident upon the light receiving element so as to suitably bias the photoelectric converting characteristic of the light receiving element by the light emanated from the light source and reflected by the walls or the like where there is no smoke in a space to be supervised. Such adjustment is one of the important factors that determine sensitivity of the detector.
The factors that determine the sensitivity and S/N ratio of the photoelectric smoke detector of the reflected light type are the quantity of the light from the light source and the quantity of light received by the light receiving element and it is desirable to use an adjusting mechanism capable of adjusting these two factors in an interlocked relationship. One solution involves the provision of independent light quantity adjusting means for the light source and the light receiving element but such solution requires two adjusting members thus complicating not only the construction of the mechanism but also the operation thereof. For this reason, in most of the prior art photoelectric smoke detectors of the reflected light type the sensitivity of the detector has been adjusted only by varying the bias voltage of the electric circuit as stated in U.S. Pat. No. 3185975, for example. However, the adjustment of the sensitivity by the adjustment of the electric circuit requires complicated and delicate skills so that more simple and more accurate adjusting means has long been desired in the art.